1. Field of the Invention
The present invention generally relates to fuel cell metallic separators and to methods for manufacturing such separators.
2. Description of the Related Art
Electrochemical fuel cell assemblies convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Electrochemical fuel cell assemblies generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes generally each comprise a porous, electrically conductive sheet material and an electrocatalyst disposed at the interface between the electrolyte and the electrode layers to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area.
Solid polymer fuel cell assemblies typically employ a membrane electrode assembly (“MEA”) consisting of a solid polymer electrolyte, or ion exchange membrane, disposed between two electrode layers. The membrane, in addition to being an ion conductive (typically proton conductive) material, also acts as a barrier for isolating the reactant (i.e., fuel and oxidant) streams from each other.
The MEA is typically interposed between two separator plates, which are substantially impermeable to the reactant fluid streams, to form a fuel cell assembly. The plates act as current collectors, provide support for the adjacent electrodes, and typically contain flow field channels for supplying reactants to the MEA or for circulating coolant. The plates are typically known as flow field plates. The fuel cell assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, as well as good sealing between fuel cell components. A plurality of fuel cell assemblies may be combined electrically, in series or in parallel, to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also separates the fluid streams of the two adjacent fuel cell assemblies. Such plates are also referred to as bipolar plates and may have flow channels for directing fuel and oxidant, or a reactant and coolant, on each major surface, respectively.
Corrosion of metallic separator plates, more specifically around the communication ports for the fluid streams, is a concern in the operation of fuel cell systems. The concern is the occurrence of a liquid short-circuit at the edges of the ports between adjacent fluid stream and the resultant electrolytic corrosion.
One way to address this concern has been through port coating, so as to provide a barrier between the liquid streams and the metallic plates. The coating solution must be electrolytically and hydrolytically stable and have a very low permeability to water vapor. However, the selection of the proper coating material has proven to be a challenge as such material must also be suitable for the high temperature and the high relative humidity environment of fuel cell systems.
Another way to address this concern is as outlined in U.S. Patent Application 2003-0143451 and Japan Patent Application 2004-039436. According to these disclosures, the edges of the metallic separators, including the communication ports, are made of thermoplastic resin and the port seals are made of silicone rubber. The method for manufacturing these plates, however, is complicated injection molding processes, as the elastomeric seals must form part of a separate step from the initial resin-forming step.
There is therefore a need for metallic separator plates which are not prone to electrolytic corrosion and which are easy to manufacture. The present invention addresses these and other needs, and provides further related advantages.